1. Field of the Invention
This invention relates to oscillator circuits and more specifically to a low power BJT-RTD oscillator.
2. Description of the Related Art
Communications systems use sinewave oscillators to establish transmitter carrier frequencies, drive mixer stages that convert signals from one frequency to another, and generate clocking signals. Of particular interest are wireless communications systems such as satellites or portable systems where low-power consumption and high frequency performance are important. Most known oscillators include a single active device such as a diode, BJT, or FET and a tuned circuit (or crystal) in a positive feedback path. The feedback loop is self limiting so that the oscillations do not become unbounded but instead reach an equilibrium at which a stable sinusoidal waveform is produced.
Solid State Radio Engineering, John Wiley & Sons, Ch. 5, pp. 128-139, 1980 describes two different types of sinewave oscillators; negative resistance and feedback oscillators. In a negative resistance oscillator, a parallel-resonant RLC circuit is connected across a negative resistance device such as an active diode. The static resistance of this device is always positive and, hence the diode always absorbs direct current. However, above a peak voltage the diode's small-signal resistance is negative. Therefore, biasing the circuit at an operating point Q above the peak voltage provides positive feedback that causes the circuit to oscillate. As long as the diode delivers more power than the load absorbs, the amplitude of the oscillations will continue to grow. On each cycle, the diode will swing further away from the operating point Q where the magnitude of the small-signal resistance becomes quite large. This reduces the power delivered by the diode until equilibrium is established.
Physics of Ouantum Electron Devices, Springer-Verlag, Federico Capasso (Ed.), pp.155-158, 1990 describes a negative resistance oscillator for use at frequencies in the hundreds of Ghz range. A waveguide is used to define the parallel-resonant RLC cavity. A resonant tunneling diode (RTD) chip is mounted on a stud that is connected to ground and connected via a "whisker" to another stud on the other side of the waveguide that is connected to a DC supply voltage. The RTD chip is biased to operate in its negative resistance region, which causes the cavity to oscillate and produce a sinusoidal waveform that travels down the waveguide to a load. A mechanical plunger is used to change the cavity dimensions and set the frequency of oscillation.
RTD chips are currently fabricated with III-V compounds such as indium phosphide (InP) or InAs/AlSb/GaSb and exhibit switching speeds in the terahertz range with peak voltages as low as 0.1V. As a result the negative-resistance waveguide oscillator uses very little power to produce a pure high frequency signal. However, because the waveguide cannot be monolithically integrated with the RTD chip, the oscillator is relatively large, expensive, difficult to mass produce and has poor reliability.
Feedback oscillators generally employ a gain element (junction transistor, FET or operational amplifier), a load (RLC circuit), and a feedback network. The output of the gain element must be fed back to its input with gain greater than unity and with a phase shift of 0.degree. or some multiple of 360.degree.. The circuit will oscillate at the frequency that produces this phase shift. Known feedback oscillators are monolithically integrated in silicon and gallium-arsenide technologies that support Ghz frequencies but require a minimum 2V supply to bias the active device. As a result, they consume a relatively large amount of power, which reduces battery life.